Author Topic: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phase...that is the question!  (Read 1308 times)

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ezee

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I am reading that 3 phase stators can be "typically 150% more efficient than single phase" according to http://www.windstuffnow.com/main/3_phase_basics.htm


I have a few questions in this regard:



  1.  Is this true regarding PMAs for wind turbines?  I mean, are 3 phase stators more efficient than single phase?  All the axial plans I have seen here and on Hugh's site seem to be single phase...just curious why (if 3 phase is more efficient) there aren't more 3 phase stator plans out there.
  2.  Can you have dual rotors with a three phase stator?  It would seem that (with the overlapping coils) having dual rotors and a three phase stator would cause some coils to be "off center" between the two rotors, or is this just not a problem?


Thanks!
« Last Edit: June 25, 2007, 08:46:46 PM by (unknown) »

kurt

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #1 on: June 25, 2007, 03:06:12 PM »
all but the very early attempts at alternators shown on the http://www.otherpower.com/ site are 3 phase and all of the alternators in hughs plans are eather 3 phase or 5 phase. i think you are confusing the fact that most of them only use a single layer of windings with single or 3 phase.
« Last Edit: June 25, 2007, 03:06:12 PM by kurt »

Ungrounded Lightning Rod

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #2 on: June 25, 2007, 03:53:29 PM »
You want polyphase (more than one phase) because single-phase produces a load that varies with the angle of the shaft, creating vibration.


Within polyphase systems, three-phase is special:  It's the one with the lowest percentage of power lost to resistive heating for a given amount of copper used in the wiring.


Polyphase doesn't require your coils in an axial-flux, coreless, permanent-magnet alternator to be overlaid - it just takes more magnets to do it than if you did overlay them.  Since overlaying the coils requires either that the coils be non-flat or the gap be increased (weakening the mag field), it's easier to fabricate a good alternator by winding flat coils and throwing in extra magnets.

« Last Edit: June 25, 2007, 03:53:29 PM by Ungrounded Lightning Rod »

ezee

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #3 on: June 25, 2007, 04:10:32 PM »
i think you are confusing the fact that most of them only use a single layer of windings with single or 3 phase.<<<I was indeed.  THanks for clarifying that!
« Last Edit: June 25, 2007, 04:10:32 PM by ezee »

ezee

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #4 on: June 25, 2007, 04:12:03 PM »
Thanks for that very thorough explanation...I was indeed thinking the three phase stators had to have layered coils.


Makes sense now, just add some magnets.


Thanks.

« Last Edit: June 25, 2007, 04:12:03 PM by ezee »

DanB

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #5 on: June 25, 2007, 06:23:53 PM »
"Makes sense now, just add some magnets."


I wouldn't simplify things quite so much - in fact it's a bit of the opposite.  You could build a single phase machine just as efficient as the 3 phase machine - it would be larger though, it would have more magnets and/or more copper in it.  With the same number of magnets you could build a 3 phase machine, or a single phase machine.  The 3 phase machine will make more power at any given rpm.


For a good 3 phase machine a nice arrangement is to use 3 coils/4 poles - that's what most folks are doing.  You could stack coils and have different arrangements - I think in the end doing so will be a pain in the neck and likely not gain you anything (it might lose you something if you're not very careful).  This is true with axial flux machines anyhow - in radial flux machines (like normal alternators)stacking coils makes a lot more sense.  It would probably make sense in an axial flux machine to stack coils if you had slotted laminations - but that stuff gets complicated for most folks to manufacture.

« Last Edit: June 25, 2007, 06:23:53 PM by DanB »
If I ever figure out what's in the box then maybe I can think outside of it.

Ungrounded Lightning Rod

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #6 on: June 25, 2007, 09:06:37 PM »
My "add more magnets" was for a single-layer 3-phase vs. an overlapped-coil 3-phase of the same number of coils.


In the single-layer version the centers of the coils are open, and a magnet over the opening is "wasted", when it would be generating on another coil-side in the overlapped version.  You make up for the "wasted" poles by having more poles.  B-)


= = = =


Throwing extra magnets at a design is a one-time bump in up-front cost.  Throwing extra copper at a design in a way that increases the fraction of copper that is outside the moving mag field doing purely interconnect duty is a not just an up-front cost for the copper but also an ongoing waste of a larger fraction of your shaft power to do resistive heating rather than battery charging / load driving.  So designs tend to focus on optimizing the coils at the cost of some extra magnets, to get more of the collected wind power into useful form.

« Last Edit: June 25, 2007, 09:06:37 PM by Ungrounded Lightning Rod »

DanB

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #7 on: June 25, 2007, 10:38:58 PM »
yes - I agree completely ungrounded.

I was just under the impression that he was starting to think that it was a requirement to add more magnet for a 3 phase machine - which isn't the case.  And yes - past a certain point adding more copper makes no sense.
« Last Edit: June 25, 2007, 10:38:58 PM by DanB »
If I ever figure out what's in the box then maybe I can think outside of it.

finnsawyer

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Re: 3 Phase or Not 3 Phase...What is the question?
« Reply #8 on: June 26, 2007, 07:44:25 AM »
If you are really interested in three phase versus single phase check out the following link:


    http://www.fieldlines.com/story/2006/4/21/16237/9933


While this does not deal with a standard single phase set up, it should provide some insight into the differences, as well as the similarities.

« Last Edit: June 26, 2007, 07:44:25 AM by finnsawyer »

Ungrounded Lightning Rod

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Re: 3 Phase or Not 3 Phase...3 Phase or Not 3 Phas
« Reply #9 on: June 26, 2007, 11:39:20 AM »
Cool.


(Just striving for clarity.)

« Last Edit: June 26, 2007, 11:39:20 AM by Ungrounded Lightning Rod »

finnsawyer

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Re: 3 Phase or Not 3 Phase...that is the question!
« Reply #10 on: June 30, 2007, 09:10:19 AM »
I finally had time to check the link.  I was not particularly impressed.  For one thing, a single phase waveform goes to zero twice during a cycle, not three times.  Also, three phase generation may be more efficient when powering three phase motors, but not necessarily when charging batteries through rectifiers.  One should beware of generalizations.  I also feel that some of the contributers misunderstand the basics.  So, I thought I'd provide my take on this.  Fell free to critique it.


For simplicity, suppose we start with 12 one inch diameter magnets and 12 coils.  The coil size is not specified at this point except to say that the coil center hole is also one inch.  We place the twelve magnets rather far apart relative to the coil size.  As a coil moves across a north pole it produces a positive voltage pulse followed immediately by a negative voltage pulse and then a time of no voltage.  Continuing over the following south pole it produces a negative pulse, a positive pulse, and a time of zero voltage.  If the magnets are brought closer together one can eliminate the times of zero voltage.  However, that still leaves a peculiar voltage waveform that goes ++,--,++,--,...  While one could probably use it, a better solution would be to bring the magnets still closer until the dual pulses combine into one pulse of greater voltage.  Well, the closest you can bring the magnets is determined by the coil size.  The sum of the magnet diameter plus the magnet spacing can be no less than the coil diameter.  For instance two inch diameter coils allow for a one inch magnet spacing.  When a coil and magnet align the voltage induced in the coil is zero even though the flux is maximum.  When the coil is exactly centered over two adjacent magnets the flux through it will be zero.  However, the flux will then be changing at a maximum rate yielding the maximum voltage.  Half the coils will be producing positive pulses and half will be producing negative pulses, which can be grouped and connected to produce a single phase output.  The output voltage will be the sum of the voltages of the 12 coils acting in phase.


If we now wish to use the same coils and magnets for a three phase alternator we start by throwing away three coils.  One would then normally move the magnets closer together so that the coils touch and that they overlap the magnets as much as possible when centered between two adjacent magnets.  A greater overlap will result in a greater induced voltage if the speed at which the magnets pass the coils doesn't change.  With the magnets closer together it does change, since a smaller diameter rotor would be used.  It's not clear that the peak voltage will actually increase.  It is clear that the average voltage will be the same in both cases.  The average voltage for a coil is given by the change in flux as the coil moves from magnet center to following magnet center divided by the time it takes.  At the same RPM with 12 equally spaced magnets the time to move 30 degrees is the same for both the single phase and the three phase alternators.  The change in flux is also the same, as the coils and magnets are the same in both cases.  At this point it should be clear that in the voltage sweepstakes the single phase will likely win, since it is getting its output from 12 in phase coils rather than two groups of three which are slightly out of phase of each other.


Another factor is alternator resistance.  The single phase will exhibit twice the resistance than the three phase.  This will affect power output.  


     

« Last Edit: June 30, 2007, 09:10:19 AM by finnsawyer »